The direct current spot welding of aluminum presents some special problems in that during welding operations a coating of aluminum tends to be deposited on the copper electrodes between which the material to be spot welded is clamped and particularly the positive electrode. This tendency to "spot" or coat limits the number of welds which can be successfully performed with a particular set of electrodes. Moreover, aluminum deposited previously on the positive electrode will contaminate a subsequent weld while also interfering with the heat and current transfer characteristics of the material. Further, due to the buildup of aluminum on the positive electrode, the underlying copper core of conventional electrode systems is depressed during the pressure welding operation and then, when the aluminum subsequently tears away, a crater mars the profile of the electrode and interferes with its welding characteristics.
When a spot of aluminum freezes or coats on an electrode it disturbs the distribution pattern of the current when the next weld is made. This occurs because the heat resistance is different at the location where the aluminum is coated on, and hot and cold spots are formed on the profiled electrodes. Normally, if electrode tips hold their contour there is an epicenter of heat application. If there is a deposition of metal on some part of the electrode tip, an uneven heat resistance is created and heat is drawn away from the weld epicenter, which then is colder than normal. The geometric symmetry of heat distribution is further disturbed if part of the aluminum is later torn off, or if a crater is formed during the pressure welding operation as a result of deformation of the copper electrode to receive the thickness of the aluminum deposit.
During a prefiling investigation which was made in this case, the following patents were cited and are made of record herein:
U.s. pat. No. 3,715,558 PA1 U.s. pat. No. 3,655,938 PA1 U.s. pat. No. 3,115,570 PA1 U.s. pat. No. 2,684,424 PA1 U.s. pat. No. 2,404,632 PA1 U.s. pat. No. 2,354,313 PA1 U.s. pat. No. 2,209,932 PA1 U.s. pat. No. 1,640,244 PA1 U.s. pat. No. 1,360,948 PA1 U.s. pat. No. 1,177,260 PA1 U.s. pat. No. 1,155,497
One of the prime objects of the present invention is to provide an electrode system which has a far greater operating life than conventional systems and can, for example, perform in the neighborhood of 5,000 aluminum welds in comparison with the 270 aluminum welds obtained with conventional systems.
A further object of the invention is to provide an electrode system in which the positive electrode is a circumferentially indexible roller which is indexed sufficiently to present a fresh peripheral surface to the work to be welded, automatically, responsive to retraction of the positive electrode to release the material welded.